Hitherto once a shape retaining material obtained by stretching a polyolefin-type resin sheet is bent, it does not return toward its original shape but retains the bent shape. Therefore, such shape retaining materials have conventionally been used for a binding sheet used in place of a metal wire, a core material of a hat brim, a mask, an apron, a bag, and the like.
Examples of the said shape retaining material proposed so far include “a method for producing a thread-like or band-like polyethylene material having plastic deformability, in which general-purpose polyethylene having a limiting viscosity of less than 3.5 dl/g is melted and extruded into a thread-like or band-like shape to form a raw yarn or band being made of the melt-solidified material of the polyethylene and having a maximum thickness of 1 mm or more, and the yarn or band is stretched at a temperature of 60° C. or higher and less than the melting point of polyethylene so that the resulting stretched product may have a return angle of 20 degrees or less 10 minutes after being bent at an angle of 180 degrees and a return angle of 15 degrees or less 10 minutes after being bent at an angle of 90 degrees” (see, for example, Patent Literature 1); and “a shape retaining material obtained by stretching an ethylene homopolymer or an ethylene-α-olefin copolymer containing 2% by weight or less of the α-olefin having 3 to 6 carbon atoms at a stretching ratio of 7 to 20 times, the homopolymer or copolymer having a density of 950 kg/m3 or more and a ratio of weight-average molecular weight (Mw) to number-average molecular weight (Mn) of 5 to 15, the material being a fibrous or band-like formed material and having a return angle of 12 degrees or less after being bent at an angle of 90 degrees” (see, for example, Patent Literature 2). In such proposals, examples of such a “shape retaining material comprising a polyethylene resin or an ethylene-α-olefin copolymer” are shown.
However, when mechanical strength in the stretching direction (MD: machine direction) and that in TD (TD: transverse direction), i.e., in a direction orthogonal to the stretching direction are compared in the said shape retaining material, the mechanical strength in the stretching direction (MD) is high and the other is not high. That is, the material is disadvantageous in that it is likely to be torn easily when pulled in TD and that it snaps when bent along the stretching direction. Moreover, the shape retaining sheet is disadvantageous in that its shape retaining properties are not likely to be exhibited in MD while exhibited in TD.
In order to remove the above disadvantages, “a shape retaining sheet in which synthetic resin sheets each having uniaxial shape retaining properties are laminated and adhered to each other so that each uniaxial direction may form a predetermined angle with adjacent ones” (see, for example, Patent Literature 3) has been proposed.
The said shape retaining sheet can have mechanical strength nearly uniform in MD direction, TD direction, and any other direction, and be imparted with non-directional shape retaining properties. However, the said shape retaining sheet, which is constituted of a plurality of shape retaining sheets laminated and adhered to one another, is disadvantageous in that single-layer shape retaining sheets large in width are required to be prepared and that the number of manufacturing steps increases, resulting in difficult production. Moreover, well known adhesion methods, such as an adhesion method using a rubber, acrylic, urethane, or silicone adhesive or pressure-sensitive adhesive, or the like; an adhesion method using a hot-melt adhesive comprising an ethylene-vinyl acetate copolymer, a linear low-density polyethylene resin, or the like; and an adhesion method in which a low-melting-point resin, such as a linear low-density polyethylene resin, is laminated between the synthetic resin sheets, and the sheets and the low-melting-point resin are heat fused are disadvantageous in that the methods require a high cost and shape retaining properties of the resulting retaining sheet are low.
Moreover, the mechanical strengths, such as tensile elastic modulus and tensile strength, of the shape retaining material are relatively low because it is produced by stretching a resin sheet chiefly composed of a polyethylene resin. Therefore, a shape retaining material having better mechanical strengths, such as tensile elastic modulus and tensile strength, has been desired.